U.S. patent number 6,330,143 [Application Number 09/512,480] was granted by the patent office on 2001-12-11 for automatic over-current protection of transistors.
This patent grant is currently assigned to Ecostar Electric Drive Systems LLC, Ford Global Technologies, Inc.. Invention is credited to Chingchi Chen, Douglas Keith Maly.
United States Patent |
6,330,143 |
Maly , et al. |
December 11, 2001 |
Automatic over-current protection of transistors
Abstract
The present invention discloses a circuit and method for
detecting a fault current in a circuit containing a power
transistor. A comparator in communication with a parasitic
inductance is utilized to sense the excessive change in current
through the power transistor. When the voltage magnitude through
the parasitic inductor exceeds a predefined limit, the comparator
triggers a gate drive circuit. The gate drive circuit is in
communication with the power transistor. The gate drive circuit
shuts off the power transistor when the predefined limit is
reached. Thus, the present invention is capable of reacting quickly
to fault currents through the power transistor thereby protecting
the transistor and other circuit components from damage.
Inventors: |
Maly; Douglas Keith (Dearborn,
MI), Chen; Chingchi (Ann Arbor, MI) |
Assignee: |
Ford Global Technologies, Inc.
(Dearborn, MI)
Ecostar Electric Drive Systems LLC (Dearborn, MI)
|
Family
ID: |
24039281 |
Appl.
No.: |
09/512,480 |
Filed: |
February 23, 2000 |
Current U.S.
Class: |
361/101;
361/86 |
Current CPC
Class: |
H03K
17/0828 (20130101); H03K 17/0826 (20130101); H03K
2217/0027 (20130101) |
Current International
Class: |
H03K
17/082 (20060101); H02H 003/18 () |
Field of
Search: |
;361/87,79,86,93.1,100-101 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sherry; Michael J.
Attorney, Agent or Firm: Mollon; Mark L. Coughlin; William
J.
Claims
What is claimed is:
1. A method for protecting a power transistor from being damaged by
a fault current, the method comprising:
providing an inductor in series with the power transistor;
detecting voltage generated by the inductor in response to a change
in current through the power transistor and the inductor, wherein
the inductor voltage varies proportionally to the change in current
through the power transistor and the inductor; and
shutting down conduction of the power transistor when the magnitude
of the inductor voltage exceeds a threshold voltage value
indicative of a fault current.
2. The method of claim 1 wherein the inductor is a parasitic
inductance inductor inherent to the power transistor.
3. The method of claim 1 wherein shutting down conduction further
comprises triggering a comparator to deactivate a gate drive
circuit.
4. A circuit for protecting a power transistor from being damaged
by a fault current, the circuit comprising:
an inductor in series with the power transistor, wherein the
inductor generates a voltage in response to a change of current
through the power transistor and the inductor, wherein the inductor
voltage varies proportionally to the change in current through the
power transistor and the inductor; and
a comparator in communication with the inductor and the power
transistor, wherein the comparator shuts down the power transistor
when the magnitude of the inductor voltage exceeds a threshold
voltage value indicative of a fault current.
5. The circuit of claim 4 wherein the inductor is a parasitic
inductance inductor inherent to the power transistor.
6. The circuit of claim 4 further comprising a gate drive circuit
which is triggered by the comparator to shut down the power
transistor when the magnitude of the inductor voltage exceeds the
threshold voltage value.
Description
TECHNICAL FIELD
The present invention relates to power transistors and methods and
circuits for protecting such transistors from damage due to fault
currents.
BACKGROUND ART
Currently, there exists numerous converters which utilize power
transistors. One application of power transistors is in
conventional power supplies. In a power supply, the power
transistors are used to switch high voltages. Such converters which
utilize power transistors may be subjected to fault currents. A
fault current through the power transistor causes serious damage to
the power transistor as well as other components of the converter.
These fault currents may be a product of short circuits or current
shoot-through.
The prior art has responded to the above-referenced problems by
offering complex protection circuits having multiple circuit
devices. Such circuits include delay protection circuits for
current shoot-through, and protection circuits for short circuits.
The addition of the referenced protection circuits are not only in
many instances slow to react to a detected fault current but are
also costly to implement. One common scheme, desat protection, must
be disabled during and shortly after switching times. Another
common scheme, phase current protection, does not measure device
current and thus cannot protect against shoot-through.
As such, a need exists for a new and improved protection circuit
for a power transistor. The new and improved protection circuit
must be capable of reacting quickly to a fault current, and must
contain minimal additional components such that the protection
circuit is low in cost without compromising reliability.
DISCLOSURE OF INVENTION
Accordingly, an object of the present invention is to provide a
method and circuit for detecting fault currents through a power
transistor circuit and providing a signal to shut down the power
transistor.
In accordance with this and other objects, the present invention
provides a circuit and method for detecting a fault current through
a power transistor. A power transistor's inherent parasitic
inductance is utilized to sense a change in current through the
power transistor. When the voltage across the parasitic inductor
exceeds a predefined limit a comparator circuit triggers a gate
drive circuit. The comparator circuit causes the gate drive circuit
to switch off the power transistor. Thus, the present invention is
capable of reacting quickly to fault currents through the power
transistor, thereby protecting the transistor and other circuit
components from damage.
Thus, in accordance with one aspect of the present invention, a
protection circuit is provided for protecting a power transistor
from damage by sensing a change in current though the power
transistor. The circuit includes a parasitic inductance inherent to
the power transistor. A voltage is generated across the parasitic
inductance when a change in current occurs through the power
transistor. The voltage varies proportionally with the change in
current through the power transistor. A comparator in communication
with the parasitic inductance and a gate drive circuit is utilized
to trigger the gate drive circuit to shut down the power transistor
when the voltage exceeds a predefined value. The present invention
contains a minimal number of components, reacts quickly to prevent
damage, and is relatively inexpensive.
In accordance with another aspect of the invention, a method is
provided for protecting a power transistor from being damaged by
sensing a change in current through the power transistor. The
method comprises detecting a change in current through the power
transistor by measuring a voltage on the existing power transistor
terminals using a voltage measurement circuit and activating a gate
drive circuit to shut down the power transistor when the voltage
exceeds a predefined value. The method according to the present
invention prevents a power transistor from being damaged by
reacting quickly to a change in current through the power
transistor wherein a large change in current is indicative of a
circuit fault.
The above object and other objects, features, and advantages of the
present invention are readily apparent from the following detailed
description of the best mode for carrying out the invention when
taken in connection with the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a circuit diagram illustrating a power transistor circuit
having a gate drive circuit connected to the base of the power
transistor, and a protection circuit connected to the gate drive
circuit for protecting the power transistor from being damaged,
according to the present invention; and
FIG. 2 is a flow diagram illustrating a method for protecting a
power transistor from being damaged by sensing a change in current
through the power transistor and activating a gate drive circuit to
shut down the power transistor when a voltage across an inductor
exceeds a predefined value, according to the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring now to the drawings, FIG. 1 illustrates a circuit 10 used
in a power supply or other device having power transistors. Circuit
10 includes a power transistor 12, a gate drive circuit 16, and a
comparator 22. Power transistor 12 includes an inherent parasitic
inductance inductor 14. Inductor 14 is in series with power
transistor 12 and thus sees the same current as the power
transistor.
Gate drive circuit 16 is a conventional gate drive circuit, as well
known to individuals of ordinary skill in the art. Gate drive
circuit 16 is connected to the gate of the power transistor 12 if
the power transistor is an insulated gate bipolar transistor (IGBT)
or the base of the power transistor if the power transistor is a
BJT. In the case of IGBTs, conduction of power transistor 12 is
controlled by the gate drive circuit 16 by providing a relatively
low power supply to the gate of the power transistor which turns on
the power transistor. Likewise, the gate drive circuit removes the
power from the gate of the transistor to shut off conduction of the
power transistor. In the case of BJTs, conduction of power
transistor 12 is controlled by the gate drive circuit 16 by
providing a current to the base of the power transistor which turns
on the power transistor. Likewise, the gate drive circuit removes
the current from the base of the transistor to shut off conduction
of the power transistor.
The collector and emmitter of power transistor 12 are connected to
a high current. The emitter connection provides access to one side
of the inductance 14. An isolated low voltage is applied to the
base (or gate) of the power transistor. The reference point for the
base (or gate) voltage is commonly labeled "emmitter" but is a
separate connector and provides access to the other side of the
inductance 14.
The voltage across inductor 14 is governed by the equation:
where V.sub.L is the voltage across inductor 14, L is the
inductance constant of inherent parasitic inductance 14, and dI/dt
is the change in current through inductor 14. When a fault occurs
in circuit 10 or in the circuit that circuit 10 is connected to, a
large change in current (dI/dt) will occur through transistor 12
and of course through inherent inductance 14. The large change in
current (dI/dt) will in turn create the voltage VL across the
inductor 14. Voltage (VL) is compared against a reference voltage
to determine if a fault current exists.
Possible faults include shoot-through and short circuits. Each of
these faults creates a large change in current through inherent
inductance 14. The large change in current immediately generates a
significant voltage (V.sub.L) across inductor 14.
Comparator 22 requires supporting circuitry as well known in the
art such as, for example, a bipolar power supply (not shown).
Further, a zener diode 24 is shown connected across the signal
sense and ground terminals as shown in FIG. 1. The zener diode
functions to protect the comparator 22 from an over-voltage. The
comparator has a negative voltage reference typically equal to -2.5
volts since the comparator circuit connections to the parasitic
inductor 14 will generate a negative voltage output signal to the
comparator.
In one embodiment, the comparator 22 is configured to trigger the
gate drive circuit 16 at a voltage of -2.5 volts when a large
change in current occurs through the power transistor 12. For
example, a fault current may have a value of 2,000 amps per
microsecond to 50,000 amps per microsecond. Thus, when inductor 14
sees a change in current at this level, the voltage output signal
will reach -2.5 volts which will trigger comparator 22. The
comparator reaches the threshold voltage (-2.5 volts) early in the
fault event, and the output of the comparator 22 will transition to
a high voltage state early on and trigger the gate drive circuit 16
to safely shut down the power transistor 12. The present invention
provides a robust protection circuit for a power transistor which
is not adversely affected by temperature changes and parameter
variations.
Reference is now made to FIG. 2, which illustrates a method for
protecting a power transistor from being damaged by fault currents,
according to the present invention. Gate drive circuit 16 applies a
voltage to the base (or gate) of power transistor 12 to bias the
power transistor 12 into conduction, as represented by block 50.
Voltage across inherent inductance 14 indicates the change in
current through transistor 12, as represented by block 52.
With continuing reference to FIG. 2, a method for protecting the
power transistor from damage is further illustrated, according to
the present invention. The voltage across inherent inductance 14 is
compared to a predefined threshold voltage, as represented by block
56. The threshold voltage is selected based on the characteristics
of a specific power transistor. In an embodiment of the present
invention, the threshold voltage is the voltage required to trigger
the comparator 22 at an early stage of a short circuit event. At
block 58, the present invention determines whether the predefined
threshold voltage has been exceeded. If the predefined threshold
voltage has been exceeded, the comparator 22 will deactivate the
gate drive circuit 16, as represented by block 60. At block 62, the
transistor is turned off by the gate drive circuit 16 after
receiving a high voltage signal from the comparator 22. However, if
the voltage across the inherent inductance 14 does not exceed the
threshold voltage in magnitude, the power transistor 12 will not be
turned off, and the change in current through the transistor will
continue to be monitored, as represented by block 52.
While embodiments of the invention have been illustrated and
described, it is not intended that these embodiments illustrate and
describe all possible forms of the invention. Rather, the words
used in the specification are words of description rather than
limitation, and it is understood that various changes may be made
without departing from the spirit and scope of the invention.
* * * * *